Image processing apparatus, image processing method, program, and recording medium

ABSTRACT

In the event of displaying only increase (or disappearance) of shadows in a difference image generated based on a display mode selected by a user, a difference image is subjected to gradient conversion so that only the luminance of the region(s) where a shadow has increased (or disappeared) differs from the luminance of other regions, and the difference image thus subjected to gradient conversion is displayed, thereby displaying the states of shadow change independently. Also, negative/positive information in a past image and a current image is extracted, and further, correlation of the diagnostic judgment reference of shadows on the difference image is input. Definition of the shadow stipulates whether the parts of the affected portion improving or deteriorating are to correspond to high concentration (high luminance level) or low concentration (low luminance level) on the difference image. This facilitates observation of changes in shadows.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image processing method, andparticularly refers to an image processing method suitably used fordisplaying change of a subject over time in medical images.

[0003] 2. Description of the Related Art

[0004] As of recent, JIRA (Japan Industries Association of RadiologicalSystems) has set the MIPS (Medical Image Processing Systems) standardsin accordance with DICOM (Digital Information and Communication inMedicine) which is an international standard regarding digitizingmedical images, and accordingly, digitizing of medical images is makingprogress.

[0005] In the medical image field, there is increased use of CAD(Computer Aided Diagnosis) for analyzing medical X-ray images, CT scanimages, MRI images, and so forth, using computers. Especially noteworthyis time-difference CAD which yields the difference over time of imagestaken of the same portion.

[0006] With CAD which handles the difference over time, image analysisis performed for a pair of plain orbital radiography images of the chesttaken at different points in time, for example. Portions in the imageswhich are anatomically the same are found, either the current or thepast image is deformed, and difference processing is performed on eachpixel.

[0007] The luminance value of the difference image obtained by suchdifference processing corresponds to the change in image signals in thecurrent and past images. That is, in the event that there is nodifference between the past image and current image, the differencevalue is 0, but in the event that there is some sort of change, there isa change in the luminance level corresponding to the change thereof.

[0008]FIGS. 22A through 22D illustrate an example of change in imagessignals for images taken of the same portion at different points intime. Signals of the past image and signals of the current image, andsignals of the difference image obtained from these, are shownone-dimensionally.

[0009]FIG. 22A shows past image signals with a generally smooth profile.On the other hand, the region A in FIG. 22B which is a current imageshows signals indicating a shadow which has appeared since. Subtractingthe signals of the current image from the past image and generatingdifference image signals of the difference yields the profile shown inFIG. 22C.

[0010]FIG. 22D is an example of a difference image of a chest X-rayimage wherein such a change has occurred, with the region A having a lowluminance corresponding to the newly-appeared shadow being shown.Displaying a difference image in this way allows changes which haveoccurred between the two images to be observed more readily, which isadvantageous, since change over time can be more readily observed fromthe two images.

[0011] However, it would be entirely mistaken to assume that shadowsonly increase or disappear from one image to another, and there arecases wherein one shadow will have increased while another hasdisappeared. That is to say, both appearance (or growth) anddisappearance (or shrinking) of shadows may be consecutively ongoingprocesses, so with the above-described technique, there is the problemthat the interpreter of the radiograms, i.e., the physician, mustcarefully observe any change in any of the images displayed in thedifference image.

[0012] Also, there is the problem that in the event that there is agreat number of shadows displayed in the difference image, change in theshadows is more readily overlooked, and moreover, with a great number ofshadows, judging the overall progress becomes difficult.

[0013] With radiograms, there are cases wherein whether high pixelvalues are displayed corresponding to high luminance or corresponding tolow luminance, depending on the image generating apparatus. For example,with apparatuses wherein digital images are generated by directly orindirectly converting X-rays which have passed through the subject intoelectric signals, as with FPDs (Flat Panel Detectors) which have comeinto practical use in recent years, image data is generated as apositive image wherein the pixel values of regions with high X-raytransmissivity such as in the lung field are high, and the high pixelvalues are displayed corresponding to high luminance on the display.Thus, the values of the corresponding pixels in the lung field are greatfor chest X-rays, for example. On the other hand, normal radiograms areobserved as negative images, so with image data wherein the X-ray filmis digitized using a film scanner or the like, the values of thecorresponding pixels in the lung field are small. Accordingly,performing difference processing with both negatives and positivestogether means that the way the shadow is displayed is not always thesame, and judgment becomes extremely difficult.

SUMMARY OF THE INVENTION

[0014] The present invention has been made in light of theabove-described problems, and accordingly, it is an object thereof toperform difference image display such that observation of change inindividual shadows can be made in an easy and sure manner even in theevent that changes in shadows are occurring at multiple locations, andalso to allow judgment of observation of the process to be made in aneasy and sure manner.

[0015] It is another object of the present invention to provide a methodand apparatus for generating consistent difference images even whengenerating difference images from negative and positive imagesintermingled, and particularly to provide a method and apparatus capableof outputting difference images with consistency in the display formatof shadows on the difference image.

[0016] According to the present invention, the foregoing objects areattained by providing an image processing method comprising: an inputstep, of inputting at least two medical images taken at different pointsin time; a difference image generating step, of generating a differenceimage by positioning two medical images input in the input step andsubjecting image signals at corresponding coordinate points todifference processing; a difference image storing step, of storing thedifference image generated in the difference image generating step in astorage medium; a reference difference value deciding step, of decidinga reference difference value in the difference image stored in thedifference image storing step; a state selecting step, of allowing auser to select a state of shadow change to serve as an indicator ofinterpreting the difference image generated in the difference imagegenerating step; a difference image processing step, of processing thedifference image based on the reference difference value decided in thereference difference value deciding step and the state of shadow changeselected in the state selecting step; and an output step, of outputtingthe difference image processed in the difference image processing step.

[0017] Further, the foregoing objects are attained by providing an imageprocessing method for generating a difference image from a first imageand a second image, the method comprising: a control step, of deciding agradient processing method such that shadow change regions are displayedin a predetermined manner based on image attributes of the first imageand the second image; a gradient processing step, of performingpredetermined gradient processing on the first image and/or the secondimage following the decision of the control step; and a computing step,of computing a difference image from the first image and the secondimage processed in the gradient processing step.

[0018] Further objects, features and advantages of the present inventionwill become apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate embodiments of theinvention and, together with the descriptions, serve to explain theprinciple of the invention.

[0020]FIG. 1 is a diagram illustrating a basic configuration of an imageprocessing device according to a first embodiment of the presentinvention.

[0021]FIG. 2 is a flowchart illustrating an example of detailed actionsof the members of the image processing device according to the firstembodiment of the present invention, i.e., the image processing method.

[0022]FIG. 3 is a diagram illustrating a histogram of a difference imageaccording to the first embodiment of the present invention.

[0023]FIG. 4 is a diagram illustrating an example of a monitor displaymade on a display unit with the first embodiment of the presentinvention.

[0024]FIGS. 5A through 5C are diagrams illustrating examples of gradientconversion properties for each mode according to the first embodiment ofthe present invention.

[0025]FIGS. 6A through 6C are diagrams illustrating examples of actualimages displayed on the display unit with the first embodiment of thepresent invention.

[0026]FIG. 7 is a diagram illustrating a basic configuration of an imageprocessing device according to a second embodiment of the presentinvention.

[0027]FIG. 8 is a flowchart illustrating an example of detailed actionsof the members of the image processing device according to the secondembodiment of the present invention, i.e., the image processing method.

[0028]FIG. 9 is a flowchart illustrating even more detailed actions ofan analysis unit according to the second embodiment of the presentinvention.

[0029]FIG. 10 is a diagram illustrating an example of a monitor displaymade on a display unit with the second embodiment of the presentinvention.

[0030]FIGS. 11A and 11B are diagram illustrating an example of gradientconversion properties for each mode according to a third embodiment ofthe present invention.

[0031]FIG. 12 is a block diagram illustrating the configuration of amedical image processing apparatus according to a fourth embodiment ofthe present invention.

[0032]FIG. 13 is a block diagram illustrating the internal configurationof the medical image processing apparatus according to the fourthembodiment of the present invention.

[0033]FIG. 14 is a block diagram illustrating the functionalconfiguration of the medical image processing apparatus according to thefourth embodiment of the present invention.

[0034]FIGS. 15A through 15C are diagrams for use in describing look-uptable properties.

[0035]FIGS. 16A and 16B are diagrams for use in describing a combinationof look-up tables.

[0036]FIG. 17 is a diagram for use in describing shadow definitions indifference images.

[0037]FIG. 18 is a flowchart for use in describing the actions of apositioning unit.

[0038]FIG. 19 is an explanatory diagram of the change in profiles ofimage signals.

[0039]FIG. 20 is a block diagram illustrating the functionalconfiguration of a medical image processing apparatus according to afifth embodiment of the present invention.

[0040]FIG. 21 is a block diagram illustrating the functionalconfiguration of a modification of the fifth embodiment of the presentinvention.

[0041]FIGS. 22A through 22D are diagrams describing an example of changein images signals for images taken of the same portion at differentpoints in time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] The preferred embodiments of the present invention will bedescribed in detail in accordance with the accompanying drawings.

[0043] First Embodiment

[0044] A first embodiment of the present invention will now be describedwith reference to the drawings. FIG. 1 is a diagram illustrating a basicconfiguration of an image processing device to which the firstembodiment of the present invention will be applied.

[0045] To begin with, the overall operations will be described. At leasttwo images input to an image input unit 1 are subjected to processing ata difference processing unit 2, and a difference image is generated. Thedifference image is temporarily stored in a memory unit 3, subjected tonecessary processing at a gradient conversion unit 4, and output fordisplay on a display unit 5. The display unit 5 is a display device suchas a high-resolution CRT monitor or liquid crystal monitor or the like,where the physician performs the diagnosis.

[0046] Also, in the following description, the pair of images serving asthe basis for the difference processing are also stored in the memoryunit 3 in the same way and output for display on the display unit 5 asappropriate, to be used for diagnosis along with the difference image.

[0047] Next, an example of the detailed operations of the members of theimage processing apparatus according to the present embodiment, i.e.,the image processing method, will be described based on the flowchartshown in FIG. 2.

[0048] [Step S100]

[0049] First, in step S100, a pair of images are output from the imageinput unit 1. In FIG. 1, the image input unit 1 is a device forinputting multiple images taken by an unshown image-taking device, andthe pair of images to be supplied for difference processing are inputfrom an external storage device such as a hard disk, for example.

[0050] Now, while the present embodiment assumes that the pair of imagesto be subjected to difference processing are taken by a predeterminedimage-taking device at different points in time and stored beforehand,the present invention is not restricted to this arrangement. Forexample, an arrangement may be made wherein one of the images hasalready been taken and stored in an external storage device, but theother image is input through the image input unit 1 as soon as it istaken. Further, the image input unit 1 may be an interface device whichinputs images stored in an external storage device connected via anetwork.

[0051] [Steps S200 and S300]

[0052] The pair of images input in step S100 are subjected topositioning and difference processing at the difference processing unit2. This generates the difference image, which is stored in the memoryunit 3. Regarding the details of the positioning and differenceprocessing of the pair of images may be performed with a knowntechnique, for example, the technique disclosed in Japanese PatentLaid-Open No. 7-37074, there entire disclosure of which is incorporatedherein by reference.

[0053] Specifically, the images are positioned by non-linearlydistorting one of the pair of digitized images, and performingsubtraction between the one distorted image and the other undistortedimage, thereby generating the difference image.

[0054] With the present embodiment, the values indicating the change inshadow that has been generated are as such that the negative valuecorresponds to a case wherein the shadow has increased, and the positivevalue corresponds to a case wherein the shadow has disappeared. However,this relation is not indispensable for the present invention, and theopposite may be used.

[0055] [Step S400]

[0056] A reference level deciding unit 6 analyzes the difference imagestored in the memory 3, decides a reference level for the differenceimage, and stores this in (unshown) internal memory. The reference levelis a value corresponding to the luminance of a pixel in a portion wherethere has been no change between the two images, and is normally 0 indifference images.

[0057] However, in the event that there is a margin or an error in thepositioning, or the image-taking conditions for the two images are notthe same, such that there is a difference in the average luminance ofthe pair of images, the reference level may not be 0. Accordingly, withthe present embodiment, the reference level is determined based on ahistogram of the difference image.

[0058]FIG. 3 is a diagram illustrating an example of a histogram of adifference image. In time-difference processing, positioning isperformed between two images, so the frequency of occurrence of aluminance level is the highest corresponding to portions with no change.Accordingly, the reference level deciding unit 6 obtains the luminancevalue Cs which has the highest frequency of occurrence in the histogram30, and stores this luminance value Cs as the reference level Cs.

[0059] [Step S500]

[0060] A display mode selecting unit 7 allows the user to select byinputting what sort of shadow change is to be displayed at the time ofdisplaying the difference image. While three types of shadow change,i.e., no change, increase in shadow, and disappearance of shadow, can beconceived, the user can select one of the following three display modesfrom the display mode selecting unit 7. (As used herein, “disappearance”includes a lessening of a shadow, as well as the complete elimination ofa shadow.)

[0061] (1) Display all shadow changes

[0062] (2) Display only increases in shadows

[0063] (3) Display only disappearances of shadows

[0064] A user interface is used for selection of the display mode, sothat the user can exclusively select one of the three modes on a screencapable of interactive input.

[0065]FIG. 4 is a diagram illustrating an example of a monitor screendisplayed on the display unit 5 upon the user making a selection fromthe display mode selecting unit 7. In this example, the difference imageis displayed on a monitor equivalent to the display unit 5, and thedisplay mode can be selected on the same screen where the differenceimage is displayed. Or, the display mode may be selected on a separatemonitor from that where the difference image is, with switches or thelike provided separately from the screen shown in FIG. 4.

[0066] With the example shown in FIG. 4, three buttons denoted byreference numerals 41 through 43 are displayed on the screen, and onlyone of these can be selected. Thus, whether to display all shadows, orto display only one of the increasing or only the disappeared shadows,is selected, and the results of this selection are output to thegradient conversion unit 4. The arrow shown in the screen in FIG. 4 isthe mouse cursor.

[0067] [Step S600]

[0068] The gradient conversion unit 4 converts the difference imagestored in the memory unit 3 based on the display mode input from thedisplay mode selecting unit 7, and outputs the converted image to thedisplay unit 5. The following is a description of each of the conversionmethods corresponding to the display modes.

[0069]FIGS. 5A through 5C illustrate an example of the relation betweenthe input luminance and the output luminance, i.e., the gradientconversion properties, for each of the display modes.

[0070] In the event that the mode is the mode for displaying all shadowchanges, the gradient conversion unit 4 converts the difference imagebased on the relation shown in FIG. 5A, and outputs the converted image.In the event that the mode is the mode for displaying only increases inthe shadows, the gradient conversion unit 4 converts the differenceimage based on the relation shown in FIG. 5B, and outputs the convertedimage. In the event that the mode is the mode for displaying onlydisappearances in the shadows, the gradient conversion unit 4 convertsthe difference image based on the relation shown in FIG. 5C, and outputsthe converted image.

[0071] In the drawings, Smin and Smax indicate the minimum and maximumvalues in the input, i.e., in the difference image. In the same say,Dmin and Dmax indicate the minimum and maximum values in the output,i.e., the output image to be displayed. Cd represents the intermediatevalue of the luminance level in the output image, and this correspondsto the reference level Cs in the difference image as described above.

[0072] Accordingly, upon the gradient conversion being performed, in theevent that the mode is for displaying all shadow changes, the differenceimage is simply linearly mapped within the luminance range of the outputimage. In the event that the mode is for displaying only increases inthe shadows, all luminance levels greater than the reference level Csare mapped to the intermediate value Cd. In the event that the mode isfor displaying only disappearances in the shadows, all luminance levelssmaller than the reference level Cs are mapped to the intermediate valueCd.

[0073] Now, the intermediate value Cd corresponds to the luminance levelof portions wherein there was no change in the original differenceimage, so only the changes in shadows where there are increases areobserved with the mode for displaying increases, and only the changes inshadows where there are disappearances are observed with the mode fordisplaying disappearances.

[0074] [Step S700]

[0075] The display unit 5 inputs and displays the difference imagesubjected to gradient conversion at the gradient conversion unit 4 asdescribed above. As for the display format, one or the other of the pastand current images, and the difference image subjected to gradientconversion may be displayed side-by-side as shown in FIG. 4 for example,or all three of these images may be displayed side-by-side. Note thatthe display format is not restricted to either of these examples, andthat various formats can be used.

[0076]FIGS. 6A through 6C are diagrams illustrating examples of actualimages displayed on the display unit 5, FIGS. 6A, 6B, and 6Ccorresponding to the mode for displaying all shadow changes, the modefor displaying only increases in the shadows, and the mode fordisplaying only disappearances in the shadows, respectively. Performinggradient conversion as described above allows the changes in shadows tobe individually displayed and observed as shown in FIGS. 6A through 6C.

[0077] Thus, with the present embodiment, in the event of displayingonly increases (disappearances) in the shadows, the difference image isprocessed and displayed such that the luminance of the regions whereinthe shadows have disappeared (increased) is mapped to the intermediatevalue Cd corresponding to the reference level Cs, so only the regionswherein the shadows have increased (disappeared) are distinguished fromother regions. Accordingly, even in the event that regions withincreases in the shadows and regions with disappearances in the shadowsexist together, with a great number of changes in shadows, each state ofincrease or disappearance can be separately displayed as an image.Accordingly, changes in the shadows can be readily observed,facilitating judgment in following developments of lesions and so forth.As a result, the physician who interprets the image can easily andaccurately interpret the image without missing increases ordisappearances in shadows, and further can easily and accurately graspthe overall change in the difference image.

[0078] Now, with the present embodiment, in the event of displaying onlyincreases (disappearances) in the shadows, the luminance of the regionswherein the shadows have disappeared (increased) is mapped to theintermediate value Cd corresponding to the reference level Cs, andconverted to the luminance of the state where there is no change in theshadows, but there is no need always to convert disappearances(increases) in the shadows to the luminance of the state where there isno change in the shadows, as long as only the regions wherein theshadows have disappeared (increased) are distinguished from otherregions. For example, the contrast of the disappearances (increases) inthe shadows may be relatively reduced.

[0079] Second Embodiment

[0080] Next, the second embodiment of the present invention will bedescribed. With the present embodiment, the extent of increase and/ordisappearance of shadows is displayed by analyzing portions where thereis change in shadows, following the gradient conversion described in thefirst embodiment. Thus, with the present embodiment, only the processingfollowing gradient conversion differs from that of the first embodiment,so portions which are the same as the first embodiment will be denotedwith the same reference numerals as those in FIGS. 1 through 6C, anddetailed description thereof will be omitted.

[0081]FIG. 7 illustrates an example of a basic configuration of an imageprocessing apparatus to which the present embodiment is applied. In thedrawing, the image processing apparatus is the same as that according tothe first embodiment shown in FIG. 1 other than an analysis unit 8 beingadded. The output image generated by the gradient conversion unit 4 isinput to the display unit 5 and to the analysis unit 8.

[0082] An example of the detailed actions of the members of the imageprocessing device according to the present embodiment, i.e., the imageprocessing method, will be described based on the flowchart shown inFIG. 8. However, all of the steps in FIG. 8 are the same as those in thefirst embodiment (shown in FIG. 2) except for step S800, so descriptionof all steps other than step S800 will be omitted here.

[0083] [Step S800]

[0084] The difference image subjected to gradient conversion at thegradient conversion unit 4, i.e., the output image, is binarized by theanalysis unit 8, where analysis is performed separately for the mode fordisplaying only increases in the shadows and the mode for displayingonly disappearances in the shadows. More specifically, characteristicssuch as the number of regions representing change, area, and so forth,are calculated separately for each display mode.

[0085] Next, the detailed operations of the analysis unit 8 will bedescribed with reference to the flowchart shown in FIG. 9.

[0086] [Step S801]

[0087] The analysis unit 8 inputs the current display mode from thedisplay mode selecting unit 7, and in the event that the display mode isthe mode for displaying all shadow changes, the processing ends. In thiscase, the output from the analysis unit 8 is invalid, but the fact thatthe output from the analysis unit 8 is invalid can be recognized by thefollowing display unit 5 by appropriating a special value. Or, anarrangement may be made wherein the display unit 5 recognizes thedisplay mode, and in the event that the display unit 5 recognizes thedisplay mode to be the mode for displaying all shadow changes, the inputfrom the analysis unit 8 is invalidated (ignored).

[0088] [Steps S802 and S803]

[0089] Further, in the event that the current mode is the mode fordisplaying only increases in the shadows, the gradient of the differenceimage following gradient conversion is inverted, and the flow proceedsto the next step. Thus, the portions where change in shadows hasoccurred in the later-described binarization processing can be uniformlyassigned white pixels.

[0090] [Step S804]

[0091] The analysis unit 8 sets a threshold value for binarizationregarding the post-gradient-conversion difference image that has beeninput. Known techniques can be used for setting the threshold, such asthe P-tile method. The settings for area ratio in the P-tile methodshould be set beforehand, according to sensitivity to change in shadows.

[0092] [Step S805]

[0093] The analysis unit 8 binarizes the post-gradient-conversiondifference image and generates a binary image, based on the thresholdvalue set in step S804. Accordingly, a binary image can be obtainedwherein portions having change in the shadows (increase ordisappearance) are assigned 1 (white), and other portions are assigned 0(black).

[0094] [Step S806]

[0095] The binary image generated in step S805 is subjected to labeling,so as to perform region separation of independent shadow changes, andthe number of the separated regions is counted.

[0096] [Step S807]

[0097] The areas of each the regions separated by the labeling performedin step S806 are calculated, and the sum thereof is calculated.

[0098] [Step S808]

[0099] The number of regions obtained in steps S806 and S807, and thesum of the area of each, are output.

[0100]FIG. 10 is a diagram illustrating an example of a monitor screendisplayed on the display unit 5 according to the processing describedabove according to the present embodiment.

[0101] In this example, the number and area of regions with change inthe shadows in the selected display mode are displayed within a displayarea 1002 below a difference image display area (an area wherein thedifference image subjected to gradient conversion is displayed) 1001.Thus, how much change is occurring at each shadow change region can bequantitatively understood.

[0102] As described above, with the present embodiment, the number andarea of regions with change in shadows are calculated and displayed, soin addition to the advantages of the first embodiment, the interpretercan quantitatively understand the state of change, and can grasp thechange in the overall difference image in an easy and sure manner.

[0103] While the above description has the area of each of the regionsdivided by the labeling to be totaled and displayed, the area for eachregion may be calculated and separately listed in the display area 1002.

[0104] Third Embodiment

[0105] Next, the third embodiment of the present invention will bedescribed. With the present embodiment, only the gradient conversionprocessing differs from that of the first and second embodiments, soportions which are the same as the first embodiment will be denoted withthe same reference numerals as those in FIGS. 1 through 10, and detaileddescription thereof will be omitted.

[0106] While gradient conversion was made with the gradient conversionunit 4 in the first embodiment such that the relation of outputluminance values as to the input luminance values (see FIG. 5) islinear, the gradient conversion does not need to be limited to thismethod. An example of gradient conversion properties according to thepresent embodiment is illustrated in FIGS. 11A and 11B.

[0107]FIG. 11A illustrates the properties in the case that the displaymode is for displaying only increases in the shadows. As indicated bythe property 1101 a, the gradient of input at the reference level Cs orlower is non-linearly converted, while the gradient of input exceedingthe reference level Cs is converted to the intermediate value Cd. Thus,output is suppressed for regions with little value change in shadows, sonoise due to shifting of position and so forth can be suppressed. Thisallows the change in shadows to be viewed more readily.

[0108] Also, FIG. 11B illustrates the properties in the case that thedisplay mode is for displaying only disappearances in the shadows. Asindicated by the property 1102 a, the gradient of input at the referencelevel Cs or higher is non-linearly converted, while the gradient ofinput below the reference level Cs is converted to the intermediatevalue Cd. Thus, output is suppressed for regions with little valuechange in shadows in the same was as above, allowing the change inshadows to be viewed more readily.

[0109] Further, gradient conversion may be performed as indicated by theproperties 1101 b and 1102 b (the dotted lines in FIGS. 11A and 11B).This allows the existence of regions from which a shadow has disappearedto be recognized at a low contrast in the event that the display mode isthe mode for displaying the increased shadows, so the relation betweenthe increase and disappearance can be readily understood at the sametime.

[0110] While the first through third embodiments have one mode selectedfrom the three display modes and one difference image subjected togradient conversion, i.e., one output image, is displayed, as shown inFIG. 4, but in the event that the display region of the display unit 5is sufficiently large, a display format may be provided for selection toallow the mode for displaying all changes in shadows and the other twomodes to all be displayed.

[0111] Also, a user interface may be provided which would allow the userto change the gradient conversion properties described in the thirdembodiment. That is to say, the user may select a desired property fromthose shown in FIGS. 5A through 5C and 11A and 11B, so that thedifference image will be subjected to gradient conversion according tothe property selected by the user.

[0112] Fourth Embodiment

[0113] In FIG. 12, a medical image processing apparatus 110 which is thefourth embodiment of the present invention is connected to medical imagegenerating apparatuses 130, 140, and 150, and a medical data server 160,via a local area network (LAN) 100. The medical image generatingapparatuses 130 and 140 are a CT scan apparatus and MRI apparatus forexample, and the medical image generating apparatus 150 is an X-rayimaging apparatus, for example.

[0114] The medical images generated at the medical image generatingapparatuses 130, 140, and 150 are directly sent to the medical imageprocessing apparatus 110, or temporarily stored in the medical dataserver 160 and then sent to the medical image processing apparatus 110.The medical images sent directly to the medical image processingapparatus 110 are stored in a storage medium of the medical imageprocessing apparatus 110.

[0115] The medical image processing apparatus 110 comprises a medicalimage high-definition monitor 120, which can display high-definitionmedical images. Also, a medical image processing apparatus 180comprising a less expensive personal computer monitor 170 may be used.

[0116] In FIG. 13, the medical image processing apparatus 110 comprisesa CPU 210, RAM 220, ROM 230, communication interface 240, and inputmeans 260, connected to a bus 200, with output devices such as a medicalhigh-definition monitor 120, a printer 250, and the like connected tothe bus 200 via suitable interfaces. The input means include a keyboard,pointing device, and so forth.

[0117] The CPU 210 controls the entire medical image processingapparatus 110 and the output devices, with control programs thereofbeing stored in the ROM 230. The communication interface 240 controlscommunication over the LAN 100, so that medical images and other dataare exchanged between the medical image generating apparatuses 130, 140,and 150, and the medical data server 160.

[0118]FIG. 14 is a block diagram describing the functional configurationof the medical image processing apparatus, FIGS. 15A through 15C aregraphs illustrating properties of look-up tables (LUT) used with themedical image processing apparatus in FIG. 14, and FIGS. 16A and 16B arediagrams illustrating a combination of look-up tables used with themedical image processing apparatus in FIG. 14.

[0119] In FIG. 14, a file of a pair of images, made up of a currentimage and a past image that have been taken of the same portion of ahuman body at different times, is input to an image input unit 1 fromthe medical data server 160 via the local area network 100. Now, theimage files are made up of radiogram data and tag information forkeeping accessory information, including whether the image is a positiveimage or a negative image.

[0120] Information indicating whether the image is a positive image or anegative image is defined by an attribute value of the medical imagefile standard DICOM format called “Photometric Interpretation”, whereinan attribute value of “MONOCHROME I” indicates a negative image and“MONOCHROME II” indicates a positive image.

[0121] The image input unit 1 outputs image data to a differenceprocessing unit 2, and extracts the information indicating whether thepast image and current image output to the difference processing unit 2are negative images or positive images, and inputs a determinationsignal SO to a control unit 10.

[0122] On the other hand, correlation of the shadow on the differenceimage and the diagnostic judgment reference is input from a shadowdefinition input unit 11, from unshown external input, such as from anexternal storage device of a computer for example, or directly from theuser. Definition of the shadow stipulates whether the parts of theaffected portion increasing or decreasing are to correspond to highconcentration (high luminance level) or low concentration (low luminancelevel) on the difference image.

[0123]FIG. 17 is a diagram illustrating an example of shadow definition,wherein Type A is defined as a region where increase of the shadow onthe difference image is a low-concentration (black) area and reductionof the shadow on the difference image is a high-concentration (white)area, while Type B is defined as a region where increase of the shadowon the difference image is a high-concentration (white) area andreduction of the shadow on the difference image is a low-concentration(black) area. Now, low concentration or high concentration indicateswhether a value is high or low as to a reference of the luminance levelof portions with no change on the difference image.

[0124] The difference processing unit 2 has look-up tables LUT1 and LUT2regarding the past and current images input from the image input unit 1,and necessary conversion processing is made at these look-up tables LUT1and LUT2. The output of the look-up tables LUT1 and LUT2 is input to alevel correction unit 203, the concentration of one image is shifted sothat the center of the histogram of one of the images, past or current,matches the center of the histogram of the other image. Positioningimages which have been subjected to level correction can improvepositioning precision.

[0125] The output of the level correction unit 203 is input to apositioning unit 20, and subjected to coordinates conversion so that thecoordinates of corresponding pixels match. The following steps shown inthe flowchart in FIG. 18 are executed at the positioning unit 20 for thecurrent and past images, so that anatomically corresponding pointsmatch.

[0126] [Step S201]

[0127] The position of the subject is detected in the current and pastimages, and the flow proceeds to step S202.

[0128] [Step S202]

[0129] Multiple regions of interest ROI are set atgenerally-corresponding positions of the current and past images, andthe flow proceeds to step S203.

[0130] [Step S203]

[0131] The current and past images are matched for each ROI. Formatching, cross-correlation coefficients are calculated, for example,and the position with the highest match is obtained as a shift vector.

[0132] [Step S204]

[0133] The shift vector obtained in step S203 is two-dimensionallyinterpolated, and parameters for coordinates conversion for matching thecorresponding coordinates of the images are calculated.

[0134] [Step S205]

[0135] Either the current or past image is subjected to coordinatesconversion based on the parameters obtained in step S204.

[0136] [Step S206]

[0137] The image subjected to coordinates conversion and the image notsubjected to coordinates conversion are output.

[0138] Next, the output from the positioning unit 20 corresponding tothe past image and the current image are each input to the look-up tableLUT3 and the look-up table LUT4, and necessary conversion is performed.The output from the look-up table LUT3 and the look-up table LUT4 isinput to an adding unit 202, where addition for yielding the differenceis performed.

[0139] The look-up tables LUT1 through LUT4 are connected to the controlunit 10, and the image input unit 1 and the shadow definition input unit11 are connected to the control unit 10. A determination signal SOindicating whether the past and current images are negative images orpositive images are input to the control unit 10 from the image inputunit 1, and information indicating whether the determination method isType A or Type B is input from the shadow definition input unit 11.

[0140] The look-up table LUT1 and the look-up table LUT2 unify bothimages to either negative or positive, and the look-up table LUT3 andlook-up table LUT4 subject one of the images to sign inversion so thatdifference is performed by addition at the adding unit 202. The addingunit 202 generates a difference images, which is input to the imageoutput unit 5.

[0141] Three types of properties can be freely selected for the look-uptables LUT1 through LUT4, as shown in FIGS. 15A through 15C. FIG. 15Ashows a non-conversion look-up table (Type I) wherein the input isoutput without conversion, FIG. 15B shows a gradient inversion(negative/positive inversion) look-up table (Type II), and FIG. 15Cshows a sign inversion look-up table (Type III).

[0142] In Table A shown in FIG. 16A, in the event that the determinationmethod is set to Type A and the past image is a negative image and thecurrent image is a positive image, Type I, Type II, Type I, and TypeIII, are set to the look-up table LUT1, look-up table LUT2, look-uptable LUT3, and look-up table LUT4, respectively.

[0143] The manner in which the past and current image data changes willbe described with reference to FIG. 19. First, as shown in tier (a) inFIG. 19, a lesion shadow which did not exist in the past image which isa negative image, exists in the current image which is a positive imagein this example. In the event that the determination method is Type A,Type II is set to the look-up table LUT2, so the current image issubjected to gradient inversion, and average value correction isperformed, so as to arrive at the state shown in the tier (b) in FIG.19.

[0144] In the event that addition is to be performed followingpositioning, sign inversion is performed on the current image in thelook-up table LUT4, whereby the data profile of the current imagearrives at the state shown in the tier (c) in FIG. 19, so difference iscalculated from addition.

[0145] That is, the current image which has been subjected to gradientinversion from a positive image to a negative image is subtracted fromthe past image which is a negative image, so in the event that theshadow has increased, this appears as a low-concentration (black) regionon the difference image, and in the event that the shadow has decreased,this appears as a high-concentration (white) region on the differenceimage.

[0146] In the event that the determination method is set to Type A, andthe past image and current image are negative images, Type I, Type I,Type I, and Type III, are set to the look-up table LUT1, look-up tableLUT2, look-up table LUT3, and look-up table LUT4, respectively.Accordingly, neither the current image or the past image is subjected togradient inversion at the look-up table LUT1 and look-up table LUT2, andsign inversion of image signals is performed for the current image atthe look-up table LUT3 and the look-up table LUT4, so the negativecurrent image is essentially subtracted from the negative past image atthe adding unit 202, whereby in the event that the shadow has increased,this appears as a low-concentration (black) region on the differenceimage, and in the event that the shadow has decreased, this appears as ahigh-concentration (white) region on the difference image.

[0147] In the event that the determination method is set to Type A, andthe past image is a positive image and the current image is a negativeimage, Type II, Type I, Type I, and Type III, are set to the look-uptable LUT1, look-up table LUT2, look-up table LUT3, and look-up tableLUT4, respectively. Accordingly, following the past image beingsubjected to gradient inversion from a positive image to a negativeimage, the current image is subtracted. Accordingly, in the event thatthe shadow has increased, this appears as a low-concentration (black)region on the difference image, and in the event that the shadow hasdecreased, this appears as a high-concentration (white) region on thedifference image.

[0148] In the event that the determination method is set to Type A, andthe past image and current image are positive images, Type I, Type I,Type III, and Type I, are set to the look-up table LUT1, look-up tableLUT2, look-up table LUT3, and look-up table LUT4, respectively.Accordingly, the past image which is a positive image is subtracted fromthe current image which is a positive image, so in the event that theshadow has increased, this appears as a low-concentration (black) regionon the difference image, and in the event that the shadow has decreased,this appears as a high-concentration (white) region on the differenceimage.

[0149] In the event that both the past image and current image arenegative images, subtracting the current image from the past image formsa difference image, and in the event that both the past image andcurrent image are positive images, subtracting the past image from thecurrent image forms a difference image, so in either case, in the eventthat the shadow has increased, this appears as a low-concentration(black) region on the difference image, and in the event that the shadowhas decreased, this appears as a high-concentration (white) region onthe difference image.

[0150] In the event that the determination method is set to Type B, andthe past image and current image are negative images, Type I, Type I,Type III, and Type I, are set to the look-up table LUT1, look-up tableLUT2, look-up table LUT3, and look-up table LUT4, respectively, in TableB of FIG. 16B. Accordingly, the negative past image is subtracted fromthe negative current image, whereby in the event that the shadow hasincreased, this appears as a high-concentration (white) region on thedifference image, and in the event that the shadow has decreased, thisappears as a low-concentration (black) region on the difference image.

[0151] In the event that the determination method is set to Type B, andthe past image is a positive image and the current image is a negativeimage, Type I, Type II, Type I, and Type III, are set to the look-uptable LUT1, look-up table LUT2, look-up table LUT3, and look-up tableLUT4, respectively. Accordingly, the current image is subjected togradient inversion from a negative image to a positive image which issubtracted from the positive past image, so in the event that the shadowhas increased, this appears as a high-concentration (white) region onthe difference image, and in the event that the shadow has decreased,this appears as a low-concentration (black) region on the differenceimage.

[0152] In the event that the determination method is set to Type B, andthe past image is a negative image and the current image is a positiveimage, Type II, Type I, Type I, and Type III, are set to the look-uptable LUT1, look-up table LUT2, look-up table LUT3, and look-up tableLUT4, respectively. Accordingly, the past image is subjected to gradientinversion from a negative image to a positive image, from which thecurrent image is subtracted, so in the event that the shadow hasincreased, this appears as a high-concentration (white) region on thedifference image, and in the event that the shadow has decreased, thisappears as a low-concentration (black) region on the difference image.

[0153] In the event that the determination method is set to Type B, andthe past image and current image are positive images, Type I, Type I,Type I, and Type III, are set to the look-up table LUT1, look-up tableLUT2, look-up table LUT3, and look-up table LUT4, respectively.Accordingly, the current image which is a positive image is subtractedfrom the past image which is a positive image, so in the event that theshadow has increased, this appears as a high-concentration (white)region on the difference image, and in the event that the shadow hasdecreased, this appears as a low-concentration (black) region on thedifference image. In the event that both the past image and currentimage are negative images, subtracting the past image from the currentimage forms a difference image, and in the event that both the pastimage and current image are positive images, subtracting the currentimage from the past image forms a difference image, so in either case,in the event that the shadow has increased, this appears as ahigh-concentration (white) region on the difference image, and in theevent that the shadow has decreased, this appears as a low-concentration(black) region on the difference image.

[0154] As described above, according to the present invention, increaseand reduction of shadows can be expressed with a single predeterminedformat, regardless of the combination of attributes of the current andthe past images.

[0155] Fifth Embodiment

[0156] With a display device capable of displaying DICOM format images,image data wherein the value of “Photometric Interpretation” is“MONOCHROME I” is displayed with the gradient inverted, and image datawherein the value of “Photometric Interpretation” is “MONOCHROME II” isdisplayed without the gradient inverted.

[0157] With the medical image processing apparatus described in thefourth embodiment, the order of subtraction was changed by changing theimage to which image signal sign inversion is performed in the eventthat both the past image and the current image have the same imageproperties, but with the present embodiment, the order of subtraction isfixed, and the same results are obtained by changing the value ofPhotometric Interpretation provided with regard to the difference image.

[0158]FIG. 20 is a block diagram describing the functional configurationof a medical image processing apparatus according to a fifth embodimentof the present invention. The configuration of the medical imageprocessing apparatus is the same as that shown in FIGS. 12 and 13, sodescription of the configuration as such will be omitted. In FIG. 18,the parts which are the same as with the fourth embodiment will bedenoted with the same reference numerals, and description thereof willbe omitted. The look-up table LUT3 and the look-up table LUT4 containedin the difference processing unit 2 are not changed in property by thecontrol unit 10, i.e., these are set so as to be unchanged.

[0159] The tag generating unit 9 generates accessory information to beattached to the difference image data output from the differenceprocessing unit 2. This accessory information includes tag informationindicating whether the difference image output from the differenceprocessing unit 2 is a negative image or a positive image. The taggenerating unit 9 generates the accessory information to be attached tothe difference image data output from the difference processing unit 2based on the combination of the accessory information of the past imagedata and the accessory information of the current image data input fromthe image input unit 1, and the shadow definition input to the shadowdefinition input unit 11. Accordingly, the difference image to be outputfrom the difference processing unit 2 is output to the image output unit5 as an image file to which accessory information generated by the taggenerating unit 9 has been attached.

[0160] Now, the fifth embodiment will be described with an examplewherein the look-up table LUT3 is fixed to Type I and the look-up tableLUT4 is fixed to Type III.

[0161] In the event that the determination method is set to Type A, andthe past image and current image are negative images, the look-up tableLUT1 is set to Type I and the look-up table LUT2 is set to Type I by thecontrol unit 10. Accordingly, neither the current image nor the pastimage is subjected to gradient inversion at the look-up table LUT1 andlook-up table LUT2. Sign inversion of image signals is performed for thecurrent image at the look-up table LUT3 which is fixed to Type I and thelook-up table LUT4 which is fixed to Type III, so the negative currentimage is essentially subtracted from the negative past image at theadding unit 202. The control unit 10 controls the tag generating unit 9so as to provide “MONOCHROME II” to the value of PhotometricInterpretation for this difference image. Accordingly, when displayingthis difference image on a display device capable of displaying DICOMimages, in the event that the shadow has increased, this appears as alow-concentration (black) region on the difference image, and in theevent that the shadow has decreased, this appears as ahigh-concentration (white) region on the difference image.

[0162] In the event that the determination method is set to Type A, andthe past image is a negative image and the current image is a positiveimage, the look-up table LUT1 and the look-up table LUT2 are set to TypeI and Type II, respectively. Type II is set to the look-up table LUT2,so the current image is subjected to gradient inversion. Sign inversionof image signals is performed for the current image at the look-up tableLUT3 which is fixed to Type I and the look-up table LUT4 which is fixedto Type III, so the negative current image is essentially subtractedfrom the negative past image at the adding unit 202. The control unit 10controls the tag generating unit 9 so as to provide “MONOCHROME II” tothe value of Photometric Interpretation for this difference image.Accordingly, when displaying this difference image on a display devicecapable of displaying DICOM images, in the event that the shadow hasincreased, this appears as a low-concentration (black) region on thedifference image, and in the event that the shadow has decreased, thisappears as a high-concentration (white) region on the difference image.

[0163] In the event that the determination method is set to Type A, andthe past image is a positive image and the current image is negativepositive image, the look-up table LUT1 and the look-up table LUT2 areset to Type II and Type I, respectively. Type II is set to the look-uptable LUT1, so the past image is subjected to gradient inversion. Signinversion of image signals is performed for the current image at thelook-up table LUT3 which is fixed to Type I and the look-up table LUT4which is fixed to Type III, so the negative current image is essentiallysubtracted from the negative past image at the adding unit 202. Thecontrol unit 10 controls the tag generating unit 9 so as to provide“MONOCHROME II” to the value of Photometric Interpretation for thisdifference image. Accordingly, when displaying this difference image ona display device capable of displaying DICOM images, in the event thatthe shadow has increased, this appears as a low-concentration (black)region on the difference image, and in the event that the shadow hasdecreased, this appears as a high-concentration (white) region on thedifference image.

[0164] On the other hand, in the event that the determination method isset to Type B, and the past image is a positive image and the currentimage is a negative image, the look-up table LUT1 and the look-up tableLUT2 are set to Type I and Type II, respectively. Type II is set to thelook-up table LUT2, so the current image is subjected to gradientinversion. Sign inversion of image signals is performed for the currentimage at the look-up table LUT3 which is fixed to Type I and the look-uptable LUT4 which is fixed to Type III, so the positive current image isessentially subtracted from the positive past image at the adding unit202. The control unit 10 controls the tag generating unit 9 so as toprovide “MONOCHROME II” to the value of Photometric Interpretation forthis difference image. Accordingly, when displaying this differenceimage on a display device capable of displaying DICOM images, in theevent that the shadow has increased, this appears as ahigh-concentration (white) region on the difference image, and in theevent that the shadow has decreased, this appears as a low-concentration(black) region on the difference image.

[0165] In the event that the determination method is set to Type B, andthe past image is a negative image and the current image is positiveimage, the look-up table LUT1 and the look-up table LUT2 are set to TypeII and Type I, respectively. Sign inversion of image signals isperformed for the current image at the look-up table LUT3 which is fixedto Type I and the look-up table LUT4 which is fixed to Type III, so thepositive current image is subtracted from the past image which has beensubjected to gradient inversion from a negative image to a positiveimage. The control unit 10 controls the tag generating unit 9 so as toprovide “MONOCHROME II” to the value of Photometric Interpretation forthis difference image. Accordingly, when displaying this differenceimage on a display device capable of displaying DICOM images, in theevent that the shadow has increased, this appears as ahigh-concentration (white) region on the difference image, and in theevent that the shadow has decreased, this appears as a low-concentration(black) region on the difference image.

[0166] In the same way, in the event that the determination method isset to Type B, and the past image and the current image are positiveimages, the look-up table LUT1 and the look-up table LUT2 are both setto Type I. Sign inversion of image signals is performed for the currentimage at the look-up table LUT3 which is fixed to Type I and the look-uptable LUT4 which is fixed to Type III, so the positive current image issubtracted from the positive past image. The control unit 10 controlsthe tag generating unit 9 so as to provide “MONOCHROME II” to the valueof Photometric Interpretation for this difference image. Accordingly,when displaying this difference image on a display device capable ofdisplaying DICOM images, in the event that the shadow has increased,this appears as a high-concentration (white) region on the differenceimage, and in the event that the shadow has decreased, this appears as alow-concentration (black) region on the difference image.

[0167] In the event that the determination method is set to Type A, andthe past image and the current image are positive images, the look-uptable LUT1 and the look-up table LUT2 are both set to Type I. Signinversion of image signals is performed for the current image at thelook-up table LUT3 which is fixed to Type I and the look-up table LUT4which is fixed to Type III, so the positive current image is essentiallysubtracted from the positive past image at the adding unit 202. Thecontrol unit 10 controls the tag generating unit 9 so as to provide“MONOCHROME I” to the value of Photometric Interpretation for thisdifference image.

[0168] Providing “MONOCHROME I” to the value of PhotometricInterpretation displays a difference image subjected to gradientinversion on a display device capable of displaying DICOM images.Accordingly, when displaying this difference image on a display devicecapable of displaying DICOM images, in the event that the shadow hasincreased, this appears as a low-concentration (black) region on thedifference image, and in the event that the shadow has decreased, thisappears as a high-concentration (white) region on the difference image.

[0169] In the event that the determination method is set to Type B, andthe past image and the current image are negative images, the look-uptable LUT1 and the look-up table LUT2 are both set to Type I, so thenegative current image is subtracted from the negative past image. Thecontrol unit 10 controls the tag generating unit 9 so as to provide“MONOCHROME I” to the value of Photometric Interpretation for thisdifference image. Accordingly, when displaying this difference image ona display device capable of displaying DICOM images, in the event thatthe shadow has increased, this appears as a high-concentration (white)region on the difference image, and in the event that the shadow hasdecreased, this appears as a low-concentration (black) region on thedifference image.

[0170] With the present embodiment, a method has been described whereinthe output of the positioning unit 201 is subjected to sign inversion atthe fixed look-up tables LUT3 and LUT4, and the conversion results areadded at the adding unit 202, but the same results can be obtained byproviding a subtracting unit 205 for subtracting the current image fromthe past image with regard to the output of the positioning unit 201, asshown in FIG. 21 which is a modification of the present embodiment.

[0171] Thus, with the present embodiment, changing the value of thePhotometric Interpretation of the DICOM format which is provided todifference images based on the shadow definition input from the shadowdefinition input unit 11 and the positive/negative information of thepast image and current image, with the order of subtraction fixed,allows the meaning of shadows to be unified for difference images ofmixed positive images and negative images.

[0172] Other Embodiments

[0173] It is to be understood that the object of the present inventioncan also be achieved by supplying a storage medium storing program codeof software for implementing the functions of the apparatus or systemaccording to the first through fifth embodiments to an apparatus orsystem so that a computer (CPU, MPU, etc.) of the apparatus or systemreads and executes the program code stored in the storage medium.

[0174] In that case, the program code itself, read from the storagemedium, achieves the functions of the first or second embodiment, andthus the storage medium storing the program code and the program codeitself constitute the present invention.

[0175] The storage medium for providing the program code may be, forexample, a ROM, a floppy (registered trademark) disk, a hard disk, anoptical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape,a non-volatile memory card, etc.

[0176] Furthermore, not only by the computer reading and executing theprogram code, but also by the computer executing part of or the entireprocess utilizing an OS, etc. running on the computer based oninstructions of the program code, the functions of the first or secondembodiment may be achieved. The latter is also one of embodiments of thepresent invention.

[0177] Furthermore, the program code read from the storage medium may bewritten to a memory of a function extension board inserted in thecomputer or a function extension unit connected to the computer. Thefunctions of the first or second embodiment may be realized by executingpart of or the entire process by a CPU, etc. of the function extensionboard or the function extension unit based on instructions of theprogram code. This is also one of embodiments of the present invention.

[0178] The present invention is also applied to a program or a storagemedium storing the program.

[0179] It is to be understood that the present invention may also beapplied to a system including a plurality of apparatuses (e.g.,radiation generating apparatuses, radiographic apparatuses, imageprocessing apparatuses, and interface apparatuses, etc.) and to a singleapparatus in which functions of these apparatuses are integrated. Whenthe present invention is applied to a system including a plurality ofapparatuses, the apparatuses communicate with one another via, forexample, electrical, optical, and/or mechanical means, and/or the like.

[0180] Furthermore, the present invention may also be applied to animage diagnosis aiding system including a network (LAN and/or WAN,etc.).

[0181] While the present invention has been described with reference towhat are presently considered to be the preferred embodiments, it is tobe understood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. An image processing method comprising: an inputstep, of inputting at least two medical images taken at different pointsin time; a difference image generating step, of generating a differenceimage by positioning two medical images input in said input step andsubjecting image signals at corresponding coordinate points todifference processing; a difference image storing step, of storing thedifference image generated in said difference image generating step in astorage medium; a reference difference value deciding step, of decidinga reference difference value in the difference image stored in saiddifference image storing step; a state selecting step, of allowing auser to select a state of shadow change to serve as an indicator ofinterpreting the difference image generated in said difference imagegenerating step; a difference image processing step, of processing thedifference image based on the reference difference value decided in saidreference difference value deciding step and the state of shadow changeselected in said state selecting step; and an output step, of outputtingthe difference image processed in said difference image processing step.2. An image processing method according to claim 1, wherein, in saidreference difference value deciding step, the reference difference valueis decided based on a histogram of difference values in the differenceimage.
 3. An image processing method according to claim 1, wherein, insaid difference image processing step, the difference value of a regionin a state not selected in said state selecting step is converted intothe reference difference value.
 4. An image processing method accordingto claim 1, wherein, in said difference image processing step, saiddifference image is subjected to gradient conversion such that a regionin a state not selected in said state selecting step is relatively lowin contrast.
 5. An image processing method according to claim 1, furthercomprising: a binary image generating step, of binarizing the differenceimage processed in said difference image processing step to generate abinary image; and a counting step, of counting the number of regionshaving a predetermined area in the binary image generated in said binaryimage generating step, wherein the processing results of said countingstep are output with the difference image in said output step.
 6. Animage processing method according to claim 1, wherein the state ofshadow change to be selected by a user in said state selecting step isselected from at least one of a state of displaying all shadows, a stateof displaying increase of a shadow, and a state of displayingdisappearance of a shadow.
 7. An image processing apparatus comprising:input means for inputting at least two medical images taken at differentpoints in time; difference image generating means for generating adifference image by positioning two medical images input by said inputmeans and subjecting image signals at corresponding coordinate points todifference processing; difference image storing means for storing thedifference image generated by said difference image generating means ina storage medium; reference difference value deciding means for decidinga reference difference value in the difference image stored by saiddifference image storing means; state selecting means for allowing auser to select a state of shadow change to serve as an indicator ofinterpreting the difference image generated by said difference imagegenerating means; difference image processing means for processing thedifference image based on the reference difference value decided by saidreference difference value deciding means and the state of shadow changeselected by said state selecting means; and output means for outputtingthe difference image processed by said difference image processingmeans.
 8. An image processing apparatus according to claim 7, whereinsaid reference difference value deciding means decides the referencedifference value based on a histogram of difference values in thedifference image.
 9. An image processing apparatus according to claim 7,wherein said difference image processing means converts the differencevalue of a region in a state not selected by said state selecting meansinto the reference difference value.
 10. An image processing apparatusaccording to claim 7, wherein said difference image processing meanssubjects the difference image to gradient conversion such that a regionin a state not selected by said state selecting means is relatively lowin contrast.
 11. An image processing apparatus according to claim 7,further comprising: binary image generating means for binarizing thedifference image processed by said difference image processing means togenerate a binary image; and counting means for counting the number ofregions having a predetermined area in the binary image generated bysaid binary image generating means, wherein the processing results ofsaid counting means are output with the difference image by said outputmeans.
 12. An image processing apparatus according to claim 7, whereinthe state of shadow change to be selected by a user by said stateselecting means is selected from at least one of a state of displayingall shadows, a state of displaying increase of a shadow, and a state ofdisplaying disappearance of a shadow.
 13. An image processing method forgenerating a difference image from a first image and a second image,said method comprising: a control step, of deciding a gradientprocessing method such that shadow change regions are displayed in apredetermined manner based on image attributes of the first image andthe second image; a gradient processing step, of performingpredetermined gradient processing on the first image and/or the secondimage following the decision in said control step; and a computing step,of computing a difference image from the first image and the secondimage processed in said gradient processing step.
 14. An imageprocessing method according to claim 13, said control step furthercomprising: an image attributes acquisition step, of acquiring imageattributes of the first image and the second image; and a shadowdefinition input step in which a user selects a display method, whereina gradient processing method is decided so as to carry out the displaymethod input in said shadow definition input step, based on the imageattributes of the first image and the second image acquired in saidimage attributes acquisition step.
 15. An image processing methodaccording to claim 13, wherein said gradient processing step includes agradient inversion step, in which gradient inversion processing isperformed on the first image and/or the second image.
 16. An imageprocessing method according to claim 13, wherein said control stepincludes an image attributes information attaching step, of decidingimage attributes of the difference image obtained in said computing stepaccording to the image attributes and the display method, and attachingthe decided image attributes to the difference image.
 17. An imageprocessing method according to claim 13, wherein the first image and thesecond image are images taken of the same portion of a human body atdifferent points in time.
 18. An image processing method according toclaim 14, wherein selecting the display method defines whether increasesor decreases in shadows in the difference image are to be represented ashigh-luminance regions or low-luminance regions.
 19. An image processingmethod according to claim 16, wherein said image attributes informationattaching step includes attaching image attributes informationrepresenting the gradient of the difference image.
 20. An imageprocessing apparatus for generating a difference image from a firstimage and a second image, said apparatus comprising: control means fordeciding an image processing method such that shadow change regions aredisplayed in a predetermined manner based on image attributes of thefirst image and the second image; gradient processing means forperforming predetermined gradient processing on the first image and/orthe second image following the decision of said control means; andcomputing means for computing a difference image from the first imageand the second image processed by said gradient processing means.
 21. Animage processing apparatus according to claim 20, said control meansfurther comprising: image attributes acquisition means for acquiringimage attributes of the first image and the second image; and shadowdefinition input means by means of which a user selects a displaymethod, wherein a processing display method is decided so as to carryout the display method input by said shadow definition input means,based on the image attributes of the first image and the second imageacquired by said image attributes acquisition means.
 22. An imageprocessing apparatus according to claim 20, wherein said gradientprocessing means includes gradient inversion means, where gradientinversion processing is performed on the first image and/or the secondimage.
 23. An image processing apparatus according to claim 20, whereinsaid control means includes image attributes information attaching meansfor deciding image attributes of the difference image obtained by saidcomputing means according to the image attributes and the displaymethod, and attaching the decided image attributes to the differenceimage.
 24. An image processing apparatus according to claim 20, whereinthe first image and the second image are images taken of the sameportion of a human body at different points in time.
 25. An imageprocessing apparatus according to claim 20, wherein selection of thedisplay method by the user defines whether increases or decreases inshadows in the difference image are to be represented as high-luminanceregions or low-luminance regions.
 26. An image processing apparatusaccording to claim 23, wherein said image attributes informationattaching means attaches image attributes information representing thegradient of the difference image.
 27. A program for causing a computerto execute the processing steps of the image processing method accordingto any one of claims 1 through 6 and claims 13 through
 19. 28. Acomputer-readable storage medium storing a program for causing acomputer to execute the processing steps of the image processing methodaccording to any one of claims 1 through 6 and claims 13 through
 19. 29.A program for causing a computer to execute the functions of the imageprocessing apparatus according to any one of claims 7 through 12 andclaims 20 through
 26. 30. A computer-readable storage medium storing aprogram for causing a computer to execute the functions of the imageprocessing apparatus according to any one of claims 7 through 12 andclaims 20 through 26.